From 03e2fa5d554204b26ba68b7761cf75fa6ed9d601 Mon Sep 17 00:00:00 2001
From: Paul Chote <pchote@users.noreply.github.com>
Date: Sun, 7 Aug 2016 19:29:03 +0100
Subject: [PATCH] Lerp turning-while-moving mobile actors along an arc.

---
 OpenRA.Mods.Common/Activities/Move/Move.cs | 41 ++++++++++++++++++++--
 1 file changed, 39 insertions(+), 2 deletions(-)

diff --git a/OpenRA.Mods.Common/Activities/Move/Move.cs b/OpenRA.Mods.Common/Activities/Move/Move.cs
index d0f637506c..2b01a2c169 100644
--- a/OpenRA.Mods.Common/Activities/Move/Move.cs
+++ b/OpenRA.Mods.Common/Activities/Move/Move.cs
@@ -292,6 +292,12 @@ namespace OpenRA.Mods.Common.Activities
 			protected readonly Move Move;
 			protected readonly WPos From, To;
 			protected readonly int FromFacing, ToFacing;
+			protected readonly WPos ArcCenter;
+			protected readonly int ArcFromLength;
+			protected readonly WAngle ArcFromAngle;
+			protected readonly int ArcToLength;
+			protected readonly WAngle ArcToAngle;
+
 			protected readonly int MoveFractionTotal;
 			protected int moveFraction;
 
@@ -304,6 +310,24 @@ namespace OpenRA.Mods.Common.Activities
 				ToFacing = toFacing;
 				moveFraction = startingFraction;
 				MoveFractionTotal = (to - from).Length;
+
+				// Calculate an elliptical arc that joins from and to
+				if (fromFacing != toFacing)
+				{
+					// The center of rotation is where the normal vectors cross
+					var u = new WVec(1024, 0, 0).Rotate(WRot.FromFacing(fromFacing));
+					var v = new WVec(1024, 0, 0).Rotate(WRot.FromFacing(toFacing));
+					var w = from - to;
+					var s = (v.Y * w.X - v.X * w.Y) * 1024 / (v.X * u.Y - v.Y * u.X);
+					var x = from.X + s * u.X / 1024;
+					var y = from.Y + s * u.Y / 1024;
+
+					ArcCenter = new WPos(x, y, 0);
+					ArcFromLength = (ArcCenter - from).HorizontalLength;
+					ArcFromAngle = (ArcCenter - from).Yaw;
+					ArcToLength = (ArcCenter - to).HorizontalLength;
+					ArcToAngle = (ArcCenter - to).Yaw;
+				}
 			}
 
 			public override void Cancel(Actor self)
@@ -344,9 +368,22 @@ namespace OpenRA.Mods.Common.Activities
 
 			void UpdateCenterLocation(Actor self, Mobile mobile)
 			{
-				// avoid division through zero
+				// Avoid division through zero
 				if (MoveFractionTotal != 0)
-					mobile.SetVisualPosition(self, WPos.Lerp(From, To, moveFraction, MoveFractionTotal));
+				{
+					WPos pos;
+					if (FromFacing != ToFacing)
+					{
+						var angle = WAngle.Lerp(ArcFromAngle, ArcToAngle, moveFraction, MoveFractionTotal);
+						var length = int2.Lerp(ArcFromLength, ArcToLength, moveFraction, MoveFractionTotal);
+						var height = int2.Lerp(From.Z, To.Z, moveFraction, MoveFractionTotal);
+						pos = ArcCenter + new WVec(0, length, height).Rotate(WRot.FromYaw(angle));
+					}
+					else
+						pos = WPos.Lerp(From, To, moveFraction, MoveFractionTotal);
+
+					mobile.SetVisualPosition(self, pos);
+				}
 				else
 					mobile.SetVisualPosition(self, To);